1. Field of the Invention
This invention relates to a stepping motor and a driving method thereof and, more particularly, relates to a composite type three-phase stepping motor having no reduction gear box for rotating directly a transfer drum of a reproducing machine or the like.
2. Description of the Prior Art
A transfer drum of a reproducing machine or the like is driven by a brushless motor. The rotary speed of the motor is reduced by using a reduction gear box from 1500 rpm to 30 or 60 rpm, for example.
FIG. 1 shows a vertically sectional front view of a conventional inner rotor thee-phase hybrid type stepping motor for driving the transfer drum or the like. FIG. 2 shows a cross section taken along lines 2xe2x80x942 of FIG. 1.
In FIGS. 1 and 2, reference numeral 11 denotes a stator, 11c a stator winding, 12 a rotor, 12G a permanent magnet, 13 a rotor shaft, and 14a1xcx9c14a6 stator magnetic poles. Symboles A1 and A2 denote one pair of stator magnetic poles for an A phase, B1, and B2 denote another pair of magnetic poles for a B phase, and C1 and C2 denote the other pair of magnetic poles for a C phase.
FIG. 3 shows a driving circuit D or Dxe2x80x2 for the three-phase hybrid type stepping motor, wherein the winding 11C1 of A phase, a winding 11C2 of B phase, and a winding 11C3 of C phase are connected to (+) and (xe2x88x92) terminals of an electric power source (not shown) through six transistors Tr1xcx9cTr6.
To each of the transistors Tr1xcx9cTr6, a base current of a predetermined value is supplied from the electric power source.
One of the transistors Tr1, Tr3, Tr5 at the upper side of said six transistors and one of the transistors Tr2, Tr4, Tr6 at the lower side of the six transistors, for example, are switched ON to supply currents to two-phase windings in the three-phase windings, respectively.
Specifically, if the transistors Tr1 and Tr4 are switched ON, the winding 11C1 of A phase becomes (+) potential and the winding 11C2 of B phase becomes (xe2x88x92) potential, and the entire sequence of energization of windings can be shown in FIGS. 4A to 4D.
FIGS. 4A to 4D show time charts indicating the operation of the driving circuit shown in FIG. 3. FIG. 4A show trigger pulses to be supplied to the transistors Tr1xcx9cTr6, respectively. FIGS. 4B, 4C and 4D show currents to be supplied to the windings 11C1 of A phase, 11C2 of B phase and 11C3 of C phase, respectively.
The above-mentioned conventional stepping motor has many problems and defects.
{circle around (1)} In the inner rotor three-phase hybrid type stepping motor as shown in FIGS. 1 and 2, if the outer diameter of the motor is about 60 mm, the resolution is less than 600 (step angle is 0.6xc2x0).
In order to obtain a resolution of more than 600, the number of the rotor pole tooth (Nr) must be increased to more than 100, so that the width of the pole tooth becomes smaller than the thickness of the magnetic iron plate forming the rotor core.
This means that the rotor cannot be formed by pressing.
When the transfer drum of the reproducing machine etc. is driven by such stepping motor having the resolution of less than 600, the fluctuation in rotation becomes large, so that the stepping motor must be rotated at a high speed and the speed must be reduced.
Further, due to the backlash etc. of the reduction member, it is difficult to enhance the color image quality of the copy formed by the reproducing machine and to prevent the color doubling from being occurred.
In order to solve such problems, it has hitherto been proposed a direct drive system wherein the rotation of the motor is directly transmitted to the transfer drum without reducing the speed.
Such direct drive system has been disclosed in the Japanese Patent Laid-Open NOS. 208668/1990 and 235573/1992.
The Japanese Patent Laid-Open NO. 208668/1990 discloses a method of rotating in a predetermined direction an electrophotographic photosensitive drum (transfer drum) by using a flat motor as a prime mover, the rotary speed of the electrophotographic photosensitive drum being controlled by a servo mechanism.
The Japanese Patent Laid-Open NO. 235573/1992 discloses a driving method of a transfer drum of a LBP wherein a brushless motor having an encoder is used as a prime mover, and the rotary speed of the brushless motor is controlled by an output of the encoder.
Another method of controlling the rotary speed of a stepping motor by an output signal of an encoder mounted on the stepping motor has been proposed.
However, either method must be carried out by using means for controlling the rotary speed of the motor, which is complicated in construction and high in cost.
On the other hand, a direct drive system simple in construction wherein a transfer drum is driven directly by a stepping motor having no encoder has been proposed.
In this system, no controlling means is required, however, the step angle of such stepping motor is 0.6xc2x0.
It is necessary to limit the fluctuation of the rotary speed of the transfer drum for a plain paper copier (PPC) or (LBP) to less than 0.2% (peak to peak) when the transfer drum is rotated at 60 rpm in order to maintain a good image quality.
An object of the present invention is to solve the above problems and to provide a composite type three-phase stepping motor and a driving method thereof functioned as a sensorless actuator, of which rotary fluctuation is small even at the low speed.
Another object of the present invention is to provide an outer rotor composite type three-phase stepping motor comprising a stator consisting of two stator elements each having 6n pieces of stator magnetic pole extending outwardly radially with Ns pieces of pole tooth formed on the tip end of each stator magnetic pole, and a permanent magnet held by the two stator elements therebetween, a rotor of magnetic material arranged so as to face to an outer periphery of the stator through an air gap, Nr pieces of pole tooth being formed on an inner periphery of the rotor, and exciting windings wound around each stator magnetic pole of the stator elements consisting of two sets of three-phase windings each wound around 3n pieces of stator magnetic pole among the 6n pieces of stator magnetic pole, an angle formed between the adjacent magnetic poles in the 3n pieces of magnetic pole with each set of three-phase windings being 60xc2x0 m/n and an angle formed between the two sets of three-phase windings being (60xc2x0 m/nxe2x88x92xcex8s), where Nr is a number of rotor pole teeth, each of n, m and Ns is an integer not less than 1, and xcex8s is a step angle and 30xc2x0 /Nr or a multiple integer of 30xc2x0 /Nr.
In case that twelve stator magnetic poles are used, one set of three-phase windings is wound around every other stator magnetic poles among the twelve stator magnetic poles, and the other set of three-phase windings is wound around remaining stator magnetic poles among the twelve stator magnetic poles, and an angle formed between the adjacent magnetic poles in the twelve magnetic poles is changed between (30xc2x0xe2x88x92xcex8s) and (30xc2x0+xcex8s) alternately.
In a driving method of the composite type three-phase stepping motor, two driving circuits each having six transistors connected to form a bridge are used and triggered alternately for driving the composite type three-phase stepping motor.
In another driving method of the composite type three-phase stepping motor having k sets of three-phase windings arranged in the circumferential direction of the stepping motor, k sets of a driving circuit each having six transistors connected to form a bridge are used and triggered in order, a first set of three-phase circuit being triggered again in (k+1)th order, where k is an integer and xe2x89xa73.
Further object of the present invention is to provide a rotor type stepping motor comprising an annular stator having 6m pieces of stator main magnetic pole extending radially with Ns pieces of pole tooth formed on the tip end of each stator main magnetic pole, and windings wound around the stator main magnetic poles, and a hybrid type rotor consisting of two magnetic rotor elements arranged so as to face to a periphery of the stator through an air gap, Nr pieces of pole tooth being formed on a periphery of each of the rotor elements, and a permanent magnet held by the two magnetic rotor elements therebetween, each pole tooth on one of the rotor elements being deviated by one half of pitch of the pole teeth from each pole tooth on the other rotor elements, the 6m pieces of stator main magnetic pole being arranged so that an angle formed between each of the adjacent stator main magnetic poles is changed and repeated m times in the order of
(1) 60xc2x0/m,
(2) 60xc2x0/m,
(3)(60xc2x0/m)xe2x88x92xcex1xc2x0,
(4) 60xc2x0/m,
(5) 60xc2x0/m,
(6)(60xc2x0/m)+xcex1xc2x0,
where m is an integer and xe2x89xa71, and xcex1 is a deflection angle (mechanical angle).
In the other case, a stator similar in construction to the above is used, and as a rotor, a cylindrical permanent magnet with N and N poles formed alternately on an outer periphery thereof is used, instead of the hybrid type rotor.
In each of the above cases, it is preferable that the deflection angle xcex1 is 30/Nr, 75/Nr, or 90/Nr, where Nr is the number of pole teeth of each rotor magnetic pole, or the number of pair of N and S poles of the cylindrical magnet.
In the driving method of the permanent magnet type stepping motor, phase windings wound around adjacent three stator main magnetic poles are connected to form a star connection or a delta connection, and excited in order by voltages, a phase of the voltage applied on the winding wound around (n)th main magnetic pole being deviated by 30xc2x0 from a phase of the voltage applied on the winding wound around (n+3)th main magnetic pole among the 6m pieces of the main magnetic poles.
In a case that 12m pieces of main magnetic pole are used, a phase difference between voltages applied on the windings wound around (n)th and (n+6)th main magnetic poles is set to 30xc2x0.
Further, it is possible to drive the permanent magnet type stepping motor as a three-phase stepping motor by setting a phase difference between the voltage applied on the winding wound around (n)th main magnetic pole and the voltage applied on the winding wound around (n+3)th main magnetic pole among the 6m pieces of the main magnetic poles, or between the voltage applied on the winding wound around (n)th main magnetic pole and the voltage applied on the winding wound around (n+6)th main magnetic pole among the 6m pieces of the main magnetic poles to zero.
The permanent magnet type stepping motor of the present invention is suitable to use as a winding type double three-phase (six phases) stepping motor, and the reactance component can be reduced to one half of that in the conventional three-phase motor of the same step angle, so that a stepping motor of high speed and high torque can be realized.
Specifically, the number of pole teeth Nr of the rotor of the same step angle according to the present invention becomes one half of that of the conventional three-phase motor, because the step angle is 180xc2x0 (Pxc2x7Nr), and the number of phase in the stepping motor according to the present invention is 6, whereas the number of phase in the conventional three-phase motor is 3.
Further, the reactance of the six phases (double three-phase) stepping motor according to the present invention becomes one half of that of the conventional three-phase motor, so that a large current can flow at a higher speed and that a higher torque can be obtained, if the speed is the same, because the reactance component which affects on the value of current in the high speed operation is expressed by Nrxc2x7xcfx890xc2x7L, where xcfx890 is mechanical angular velocity, and L is an inductance of coil.
A further object of the present invention is to solve the above task by using a stepping motor of which basic step angle is less than 0.3xc2x0.
FIG. 5 shows the relation in rotary speed and fluctuation of rotation between a hybrid type three-phase stepping motor of which basic step angle is 0.6xc2x0, suitable to drive the reproducing drum and a double three-phase stepping motor of which basic step angle is 0.3xc2x0, wherein two sets of three-phase windings are mounted on a stator.
The basic step angle xcex8s of the stepping motor can be expressed by xcex8s=180xc2x0/(Pxc2x7Nr), where P is the number of phases of the stator windings and Nr is the number of pole teeth of the rotor.
The number of pole teeth Nr of the rotor of a first three-phase stepping motor of which basic step angle is 0.6xc2x0 shown in FIG. 5 is 100. The number of pole teeth Nr of the rotor of a second double three-phase stepping motor of which basic step angle is 0.3xc2x0, wherein two sets of three-phase windings are mounted on a stator is 100.
The number of phases P of the stator windings of the first motor is 3, so that xcex8s1=180/(3xc2x7100)=0.6xc2x0, whereas the number of phases P of the stator windings of the second motor is 6, so that xcex8s2=180xc2x0/(6xc2x7100)=0.3xc2x0.
The rotary speed N of the stepping motor can be expressed by N=xcex8sxc2x7Puxc2x7(60/360) rpm, where Pu is a driving pulse number per a second.
The speed fluctuation xcex94N of the driving system using the stepping motor can be expressed by xcex94N=(xcex94T/xcfx89)xc2x7nxc2x7J, where xcfx89 is rotary angular velocity (rad/sec), n is a torque ripple number per one rotation, which can be expressed by 360/xcex8s in case of the stepping motor, xcex94T is a width of the torque fluctuation which is reduced according to the phase number of the stator windings, and J is a moment of inertia of the load.
In the direct driving system for driving directly the reproducing drum by the stepping motor under the conditions mentioned above, it may be concluded that such a stepping motor that n is large (the step angle is small) and the phase number of the stator windings is large, and thus xcex94T becomes small should be selected, when the moment of inertia J of the reproducing drum and the angular velocity xcfx89 are given.
FIG. 5 shows a property of the typical first and second motors each connected to the load J and rotated while varying the drive pulse number Pu, the rotary speed N being plotted on the abscissa and the rotary fluctuation xcex94N being plotted on the ordinate.
It is apparent from FIG. 5 that in the first motor having the three-phase stator windings, of which basic step angle is 0.6xc2x0, the rotary fluctuation of 0.2% can be realized at a rotary speed N more than 195 rpm, and the rotary fluctuation becomes so large that it cannot be measured at a rotary speed of 60 rpm, and that in the second motor having double three-phase windings, of which basic step angle is 0.3xc2x0, the rotary fluctuation of 0.2% can be realized at a rotary speed N of 60 rpm.
Yet further object of the present invention is to provide a rotor permanent magnet type stepping motor comprising a stator having twelve stator main magnetic poles extending radially, symmetrical with respect to a point to one another, and stator windings for phases wound around the stator main magnetic poles, the stator windings for each phase being wound around two stator main magnetic poles symmetrical with respect to a point to each other to form the same polarity, a rotor of magnetic material consisting of two rotor elements each having even number of pole teeth, and a permanent magnet held by the two rotor elements therebetween, each pole tooth on one of the rotor elements being deviated by one half of pitch of the pole teeth from each pole tooth on the other rotor elements, wherein an angle formed between the adjacent main magnetic poles in the twelve main magnetic poles is changed between (30xc2x0xe2x88x92xcex1xc2x0) and (30xc2x0+xcex1xc2x0) alternately, where xcex1 is a deflection angle.
In the other case, a stator similar in construction to the above is used, and as a rotor, a cylindrical permanent magnet with N and N poles formed alternately on an outer periphery thereof is used, instead of the hybrid type rotor.
In each of the above cases, it is preferable that the deflection angle xcex1 is 30xc2x0/Nr, 75xc2x0/Nr, or 90xc2x0/Nr, where Nr is the number of pole teeth of each rotor magnetic pole, or the number of pair of poles of the cylindrical magnet.
It is preferable that a plurality of pole teeth are formed on each tip end of the twelve stator main magnetic poles of the stator.
The permanent magnet type stepping motor can be driven in such a manner that first windings wound around the every other main magnetic poles, and second windings wound around the remaining main magnetic poles are connected to one another to form a star connection or a delta connection, and the first and second windings are excited by currents different in phase from each other.
Further,the permanent magnet type stepping motor can be driven as a three-phase stepping motor by exciting stator windings of adjacent two phase by currents similar in phase to each other.
The permanent magnet type stepping motor of the present invention is suitable to use as a winding type six phases stepping motor, and the reactance component can be reduced to one half of that in the conventional three-phase motor of the same step angle, so that a stepping motor of high speed and high torque can be realized.
Specifically, the number of pole teeth Nr of the rotor of the same step angle according to the present invention becomes one half of that of the conventional three-phase motor, because the step angle is 180xc2x0/(Pxc2x7Nr), where P is the number of phase.
Further, the reactance of the six phases stepping motor according to the present invention becomes one half of that of the conventional three-phase motor, so that a large current can be flowed at a higher speed and that a higher torque can be obtained, if the speed is the same, because the reactance component which affects on the value of current in the high speed operation is expressed by Nrxc2x7xcfx890xc2x7L, where xcfx890 is mechanical angular velocity, and L is an inductance of coil.
These and other objects and features of the present invention will become apparent from the following description in conjunction with the attached drawings.